In the past, there has been provided an ultrasonic sensor configured to transmit an ultrasonic wave and receive an ultrasonic wave reflected by an obstacle. Such an ultrasonic sensor may be attached to a bumper of a vehicle as disclosed in document 1 (JP 2012-122936 A), for example.
The ultrasonic sensor disclosed in the document 1 includes two mounting bases to be bonded to an inner surface of the bumper, and a sensor body holding a transceiver to transmit and receive ultrasonic waves. The respective mounting bases and the sensor body include connection parts allowing detachable connection between the mounting bases and the sensor body. To fix the ultrasonic sensor to the bumper, at first the two mounting bases are respectively attached to opposite sides of an exposing hole penetrating the bumper. Thereafter, the connection parts of the sensor body are connected to the respective mounting bases respectively, and thereby the ultrasonic sensor is fixed to the bumper.
In the above conventional example, while the ultrasonic sensor is fixed to the bumper, an ultrasonic wave transmitting and receiving surface is flush with an outer surface of the bumper. However, there are some ultrasonic sensors including a part slightly protruded from the exposing hole to conceal an inner surface of the exposing hole (attachment hole). In general ultrasonic sensors, to attenuate unwanted vibration, the transceiver is covered with a cover made of elastic material. In the above configuration, part of the cover slightly protrudes from the attachment hole.
However, in this configuration, for example, when the bumper is wiped with a towel in car washing or waxing, the towel may strike the part of the cover protruding from the attachment hole and therefore external force may be applied to such part. This may cause rolling up of part of the cover and hence a partial lift of the cover is likely to occur (part of the cover is likely to be lifted).